The present application relates generally to automobile control systems, and more particularly to systems for extricating a stuck vehicle.
The universally recognized and utilized method to free a vehicle stuck in snow or mud consists of setting up a rocking motion by judicious use of the throttle and the range selector mechanism (the shift lever). Initially, a small movement either forward or backward is initiated, followed by a movement in the opposite direction. Acceleration is timed to provide positive feedback to the cyclic movement, until the rocking motion is sufficient to free the vehicle. The harmonic nature of the vehicle response requires instinctive action by the driver to properly time the acceleration bursts. Also, skill is required to avoid over-acceleration, which results in spinning the tires and worsening the situation.
Sometimes, however, rocking the automobile does not succeed in extricating it. One reason for that result is the time required to move the shifter, along with the hydraulic response latency within the transmission. These delays may combine to make the transition time between forward and reverse drive ranges longer than the period of the resonant frequency needed to free the vehicle. Although most contemporary vehicles still have this design limitation, some now incorporate full electronic range control (e-shift), eliminating the direct mechanical connection between the range selector mechanism (traditional shift lever) and the transmission range control mechanism. Full electronic range selector mechanisms provide an electronic range “request” signal to an electronic controller that “authorizes” the request and sends a subsequent electronic signal directly to a range changing solenoid within the transmission or to an electro-mechanical device connected to the transmission range control mechanism to complete the range change. The electronic range selector mechanism allows computer intervention between the driver's movement of the shifter (shift request) and the controller's response—either inhibiting it, retiming it, or automating it in some way. In addition, in at least some of the initial e-shift applications, the potential transitional response between forward and reverse drive ranges is much faster than the manual shifter motion that can be achieved by drivers. By making the increased actuation rates of e-shift possible, full computer aided rock-cycle control using such e-shift hardware can increase vehicle extrication capability by reducing the overall shift time as needed.
Although the rocking method is instinctive and effective, the rapid shifting and often simultaneous application of engine torque can result in severe damage to the transmission if sustained too long, due to rapid heat build-up. Such user behavior is designed and tested for in each automatic transmission design, but the prospective damage makes rocking a highly discouraged behavior by most automobile manufacturers. Moreover, drivers do not receive any obvious indication about the probability of rock cycling effectiveness in a particular situation or how much heat and damage to their transmissions has resulted. This often leads to permanent damage such as burned clutch material and transmission fluid.
Some vehicles are now designed with on-board software that monitors key vehicle sensors, such as throttle, shifter, and wheel movement to identify when a vehicle is being abusively shifted or rock-cycled. Once identified, rock cycle mode controls take over to attempt prevention of vehicle damage from abusive engagements. Initial executions of these actions included only accelerator pedal based clutch pressure increases, but more recent executions include engine torque limits imposed to limit energy supplied to the clutches. The torque limits are implemented with throttle, spark, and fuel control. Often, these torque limits may become so low that it makes rock cycling difficult or essentially impossible.
Therefore, a need exists to implement automatic methods and systems for extricating a stuck vehicle while minimizing risk of damage to the vehicle.